Field notes on some Broadland churches in the Wroxham area, Norfolk

Field notes on some Broadland churches in the Wroxham area, Norfolk.

Ruth Siddall, January 2019.

 The building stones of Broadland Churches visited by Ruth Siddall and Tim Atkinson on 12 January 2019. Please regard these as field notes; they are not intended to be exhaustive or complete records of the fabric of the churches described below. 

St Peter’s, Belaugh

TG 2890 1841

12thCentury with 15thCentury alterations and 19thCentury restoration by Butterfield & Phipson.

Rubble Masonry: Coarse flint, ferricrete, Wroxham Crag[1]component (minor), beach flints (minor).

Course flints retain cortices and may be very large > 30 cm long-axis single flints (NB very large flints are also used as gate posts in the village).

Ferricrete (very dark brown – almost black, pebbly [angular, moderately sorted], friable and rarer pieces of rust-red pebbly variety, with stratified pebbles). The dark ferricrete is used for quoins on NE corner and for (filled) window dressings on the north wall and as squared blocks in the lower part of this wall. These are probably from the 12thCentury phase and the ferricretes blocks are possibly reused. Ferricrete blocks have developed a pale green-white encrusting limestone.

Wroxham Crag component: white vein quartz cobble (south wall, close to door).

15thCentury rubble masonry includes a few slabs of Barnack Stone on west wall. Clipsham Stone dressings on windows of this period?

19thCentury – Knapped flint work with Ancaster Stone dressings. Uncoursed, coarse flint with cortices. Some Conulussp. echinoids. Some coursed, square-knapped flint work on buttresses.

Interior: Font (12thCentury) – grey green, fine grained limestone with a few white bivalve fossils. Unidentified stone.

North facing wall of St Peter’s Church, Belaugh showing several phases of construction and restoration and the distinctive, almost black blocks of ferricrete.

Knapped flints from the south facing wall containing a Conulus sp. Echinoid.

Blocks of ferricrete along with unknapped, uncoursed flints in the south wall of St Peter’s Church, Belaugh. These stones have been recycled into restoration phases.

Ferricrete used as quoins and ashlar on the NE corner of the church.

 

St Helen’s, Ranworth

Woodbastwick Road, Ranworth, Norfolk NR13 6HS

TG 3560 1476

First phase 1290-1350, Second Phase (Perpendicular-style alterations) up to 1530. Rood Screen 1419. North side of the church is much colonised by lichen.

Carved plinths (lamb of God, shields) on buttresses – Clipsham or possibly Barnack Stone.

Quoins, tracery and other dressings: Clipsham Stone

Rubble Masonry: Coarse flint, ferricrete (bright red-orange), Greensand (minor), Wroxham Crag component (vein quartz, Triassic quartzites, chatter-marked flints).

Flints – coarse with intact cortices, some knapped. Probably some are field flints. Upper parts of the building, including the tower have a large amount of red-orange flints. A circular, red flint in the tower (south side, c. 12 m up) is a paramoudra. Square, knapped, coursed flint work on foundations and buttresses.

Red-stained flints on the tower at St Helen’s, Ranworth including a ring-shaped paramoudra from the local chalk

Red-stained flint with fossil echinoid.

Ferricrete is a bright orange red cemented gravel with rounded to sub-rounded, poorly sorted, flint pebbles up to 4 cm long axis. Some blocks are of a gritty sand with sparse pebbles.

Blocks of ferricrete at St Helen’s Church, Ranworth.

The ferricrete has variable texture and clast size/distribution. However, it is notably a bright. Brick red in colour.

Wroxham Crag component: cobbles of vein quartz and red-pink Triassic Bunter pebbles (fist size) are common within the fabric, as are rounded, chatter-marked beach flints.

   Vein quartz cobbles from the Wroxham Crag at St Helen’s, Ranworth.

Spolia: a few rough fragments of Barnack Stone and a fine-grained Caen Stone pilaster. Greensands (possibly Kentish Rag) occur in a few squared blocks. These are possibly spolia or ballast.

Small pillar of Caen Stone spolia.

Greensand block, probably spolia or ballast.

Interior: grave slab of Green Purbeck Marble (with Uniosp.) flanked by smaller slabs of Red Purbeck Marble. Also slabs of 18thCentury of black Lower Carboniferous limestone with small white rugose corals, sparse brachiopods and a gastropod. This is probably Belgian or Irish in origin.

 Green Purbeck Marble grave slab with thick walled, Unio sp. Bivalves inside St Helen’s Church, Ranworth.

 

St Mary’s, South Walsham

23, The Street, South Walsham NR13 6DQ

TG 3653 1325

First phase 12th-13thCentury but much is 13th-14thCentury. Tower and porch are 15thCentury. Heritage Norfolk states ‘This church has a large quantity of lava incorporated into the walls along with reused masonry.’ I have no idea what the ‘lava’ refers to!

Lower courses (C12-13) of coarse flints with cortices intact, roughly coursed. Upper parts of knapped, uncoursed flint with spolia. The latter is composed of slabs of Purbeck Marble (much weathered, but with clear Viviparussp., 10 cm thick, c. 30 cm long) occur about 1 m up from foundations in the south and east chancel walls. Also, a fragment of carved, spoliated capital in Caen Stone or Clunch to the west of the down pipe of the buttress between the nave and chancel on the south side.

Exotic pebbles: sub-angular, brown, quartz arenites. These may be derived from the Crag or tills or could be ballast. Possibly striation on one example suggests till.

Porch: Square knapped, coursed flint flushwork with Clipsham Stone dressings (15thCentury).

Unknapped, beach flints have been added to the top of the walls of the north porch, probably in in the 19thCentury.

Knapped, coarse flints with boulders of brown arenaceous sandstone at St Mary’s, South Walsham.

The photo above shows a boulder which may show evidence of glacial striae.

St Mary’s Church, South Walsham. Fragment of carved capital in Caen Stone used as spolia in the church’s fabric.

Slabs of Purbeck stone, weathering in are laid in a line across the centre of the photograph.

 

St Lawrence’s, South Walsham

The Street, South Walsham NR13 6DQ, located next door to St Mary’s (above)

A 14thCentury Church restored in 1992. This building was visited very briefly.

Knapped, uncoursed flint work with Clipsham Stone dressings.

Stonework at St Lawrence’s, South Walsham; detail of Clipsham Stone dressings; this cross-bedded calcarenite is packed with shell fragments.

Knapped flint cobbles.

 

All Saints, Hemblington

Church Lane NR13 4EF

TG 35 11

Saxon-Norman Round Tower, chancel c. 1300, nave c. 1400 with 15thCentury roof and wall-paintings (of St Christopher).

Walls are predominantly of unknapped, coursed flint with cortices intact. Some (faint) herringbone coursing. Some knapped flint.

Exotic cobbles: Abundant, well-rounded quartzite (Triassic) pebbles and also sub-rounded to sub-angular cobbles of brown arenaceous sandstones, probably from Wroxham Crag and related deposits. Also a few one-off finds i.e. Hertfordshire Puddingstone sarsen, a medium-grained diorite.

Abundant spolia in Caen Stone is present. The Tudor brick window on the tower is cut into what was probably a spoliated window sill in Caen Stone. There is also a large amount of brick and tile incorporated into the walls.

Flushwork, knapped flint crosses on either side of the south porch entrance.

Dressings on buttresses in Lincolnshire Limestone (not fully observed).

The east end of All Saints Church, Hemblington showing at least two phases of construction.

Flushwork flint crosses in the brick-built south porch.

Carved spolia and rough flints in the walls.

Carved spolia in Caen stone. This is the remains of a window sill which has been cut into by Tudor brick work (on the left of the image).

Detail of stones at All Saints Church, Hemblington. Triassic quartzite split pebble.

A medium grained diorite of unknown origin.

Hertfordshire puddingstone, a form of sarsen.

Knapped flint with the trace of a fossil echinoid.

Download these notes as a pdf

Bibliography

Candy, I., Lee, J. R. & Harrison, A. M. (Eds.), 2008, The Quaternary of northern East Anglia., Quaternary Research Association., 263 pp.

Hart, S., 2000, Flint architecture of East Anglia., Giles de la Mare Publishers Ltd., London., 150 pp.

Norfolk Heritage Explorer

[1]According to Candy et al. (2008), the pre-Anglian Wroxham Crag and associated Bytham Gravel Beds contain a clastic component dominated by pebbles and cobbles of vein quartz, Triassic quartzite, ‘schorl’, Carboniferous chert, RhaxellaChert, flint and chatter-marked flint. The Bytham River flowed from the English Midlands, eastwards into the North Sea and provided a sediment source for the Wroxham Crag. The Wroxham Crag is differentiated from the underlying Norwich Crag by the presence of these ‘far travelled pebbles’.

Inside the Bartlett Brick

The new building at 22 Gordon Street houses The Bartlett School of Architecture. Completed in 2017 and designed by Hawkins\Brown Architects, the building has recently been named on the shortlist for The Architect’s Journal AJ100 Building of the Year prize and the RIBA London Awards. The structure is a retrofit of the former Wates House, the ghost of which still exists in the core of the building. What replaces it is a spacious, fit-for-purpose building for the UCL School of Architecture.

Now those of you that know me and know that my interests in the fabric of UCL are mainly biased towards the use of stone, may have assumed that I was disappointed to see a brick and concrete structure appear on the corner of Gordon Street and Endsleigh Gardens. Not so. I declare a fondness for both materials, though I confess to prefer my concrete to be around 2000 years old, the bricks used to clad 22 Gordon Street are rather special.

I am very grateful to the Bartlett’s Kevin Jones for supplying me with a specimen of ‘The Gordon Street Klinker’ a brick made specially for the construction of the building by the German brick makers Janinhoff.

Brick

Janinhoff make a great deal of bricks of different compositions, but mainly to a standard size. The Gordon Street Klinker is slimmer and longer than the standard bricks in their repertoire. 140,000 of these bricks, measuring 290 x 52 x 70 mm were used on the façade.

The brick is water-struck and twice fired. The water-struck brick making process works well for high moisture content, high plasticity clays. The clay is ‘struck’ in a wooden mould, and the large amount of water present allows for the mould to be easily removed, without the clay sticking to it. Evidence of this process can be seen in the puckered, ‘troweled’ surface of these bricks and the lip from clay overhanging the top of the mould. Lower water content clays need to be ‘sand-struck’; i.e. the mould is coated with sand which stops the brick from sticking to it.

IMG_3026

Evidence that these bricks are hand made can be seen in this example from the Endsleigh Gardens façade of the Bartlett’s 22 Gordon Street Building …

IMG_3029

Look closely and you can see the brick maker’s fingerprints …

IMG_3030

Janinhoff fire their bricks in a circular, so-called Hoffman Kiln which can operate continuously. Patented by Friedrich Hoffmann in 1858, these are the standard brick kilns used worldwide today. They are large circular or oval structures often with a central chimney. The interior has firing chambers radiating out from the central space (below the chimney). The kilns are fired by a movable ‘fire wagon’ which can travel on rails around the interior, firing each chamber consecutively.

The Gordon Street Klinker is coal fired at maximum temperatures of 1200°C. Looking at the surfaces of the bricks, ‘firing ghosts’ are seen patterning the surface of the bricks. This tells us how they were stacked in the kiln, subtly changing the oxidation of the brick surfaces where they touch each other.

IMG_3027

ringofen_11_0

Above: Janinhoff’s Hoffmann kiln, showing bricks being stacked ready for firing and also note the use of spacers and other kiln furniture.

The grey colour of the bricks indicates a low-iron content clay. Such materials are available in the northern Germany and Denmark deposited as Quaternary glacial clays. The Jutland Peninsula lay at the southern edge of the ice sheet during the last glaciation and the clays were deposited as the ice sheet retreated between 20 – 10 thousand years ago.

To be workable, reduce shrinkage and cracking on drying and firing and be strong, brick clays need to contain inclusions or temper. Inclusions are natural mineral grains that occur naturally in the clay whereas a temper is added by the brick makers and can include both mineral and organic particles.

To look inside the Bartlett bricks it is necessary to produce a thin section, a slice 30 µm thick for observation using polarising light microscopy. This technique is routine in geology for examining rocks and identify their component minerals. As ‘synthetic stones’, ceramics may also be analysed in this way under the discipline of ceramic petrology.

Under the microscope, the Bartlett bricks have an almost isotropic (opaque) clay matrix indicating that the clay minerals have broken down and have begun to melt. This indicates that the brick has been fire in excess of 1000°C (and indeed we are aware from the manufacturers that temperatures of 1200°C were attained). It is not possible to distinguish whether or not the brick clay contained inclusions or it has been tempered, or perhaps both! Mineral particles present are dominantly quartz, but chert, feldspar and zircon are also present, indicating granitic rocks in their source. Grains are poorly sorted ranging from very fine (a few 10s of microns diameter) to particles of around 0.5 mm, the latter just visible to the naked eye. It is reasonable to expect that the finest portion are naturally occurring inclusions in these glacial clays.

The photomicrographs below show the brick photographed in plane polarised light and under crossed polars, x 40 magnification.

brick_4

The mineral grains are sub-rounded to sub-angular, indicating a natural sand source (i.e. not mechanically crushed). The composition is clearly granitic as indicated by the feldspar, zircon (this in typically tiny grains) and the large quartz grains with fluid inclusion trails. Some of these features are seen in the photomicrographs below. Left, the large grain in the centre is a feldspar; x 40 magnification, the field of view is ~ 3.5 mm. Middle and Right, quartz grains with inclusion trails, x 100, the field of view is ~ 1.5 mm.

IMG_6407

Citation

Siddall, R., 2017, Inside the Bartlett Brick, Blog: Orpiment https://orpiment.wordpress.com/2017/06/02/inside-the-bartlett-brick/

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The mis-appliance of science in cultural heritage?

Science applied to archaeology and cultural heritage is a thing. It has been happening for decades. Scientific analysis of materials can provide much needed information about materials, trade, manufacture, provenance, foodstuffs, populations, individuals. With today’s kit we can make analyses on tiny samples, or even acquire semi-quantitative analyses without the need for the destruction required to remove a sample. We can identify the components in building materials and pigments, the type of honey a vessel once held, the isotopic signature of metals and therefore their provenance, the isotopic signatures of bone material can tell us were a person lived, where they migrated, where they came from. Science can revolutionise established archaeological chronologies. Amazing information. So why is so much science applied to archaeology and cultural heritage so bad?

I have been to several conferences this year where there has been a cross- and multi-disciplinary approach to materials in cultural heritage. This if course should be a GOOD THING. However, myself and colleagues have been commenting that with the proliferation of analytical techniques and access to them, the science is actually getting worse, and this is not good. We feel that the study of science in cultural heritage is not moving forward. I also see this in papers I review. Scientists are collaborating with archeologists and art historians, but it seems that they are not communicating well and there is little effort on either side to learn about each other’s discipline and what the questions the ‘science’ aims to answer. Inappropriate analytical techniques are used and poor data are produced and these data are, again inappropriately, under- or over-interpreted. The discussions at conferences and recent press activity on the use of a synchrotron to identify the presence of calcium on a Greek vase have spurred me to write this. I admit I have only read the press releases on this latter research, and I presume that the actual publication will provide much more depth to these analysis.

I’m a geologist. My PhD used geothermochronological techniques to look at landscape evolution on a continental scale. I used a radiometric dating technique called fission track analysis which used on the mineral apatite. It was known that the chemistry of apatite, with substitutions between chlorine, fluorine and the hydroxyl group, affected the lengths of the fission tracks and therefore the thermochronometric data obtained. I looked at the subtle changes of chemistry in apatites using Fourier Transform Infra-Red spectroscopy (FTIR). Although I went on to lecture in ‘orthogeology’, much of my research has been associated with the application of geological techniques to cultural heritage. I am so glad – and lucky – to have learned how to use petrology/petrography, geochronology and spectroscopy from first principles, rather than to have stumbled across these techniques as  ‘black box’ methods to reveal more about the material I am working on. I understand what these techniques actually tell us about the materials they are applied to. They are measuring bonds and their configurations on a molecular scale, the number of undecayed v. decayed isotopes (or their proxies), elemental weights or excitation of electron energy levels. These data can then be interpreted to give us information such as a radiometric age, they may be diagnostic of the identification of a mineral (or analogous compound) or the nature and origin of an organic compound, they can elucidate the presence or absence of a certain element or the precise chemical compound present in a sample.

The important word here is interpreted. Yep, that’s right, some guy sits down and writes a computer programme to interpret the results that a machine turns out. This is what the software that comes with your new black box analytical machine is. It is NOT the machine telling you the answers. So if you click on the sample image in your back-scattered electron image of your sample (yes, that’s right it is not a photo) and your spectra tells you that a peak is assigned to silicon, this is a programmers interpretation of what fits that peak. Now of course the vast majority of these analyses will be correct and completely reliable, but they should still be used with caution. I have always found pertinent this quote from the 1990 film The Hunt for Red October, in which a submarine sonar operator Jones (played by Courtney B. Vance) discovers that his interpretation software is not telling him the truth. He has detected the faint but rhythmic sounds of another submarine which is using a new propulsion mechanism and is trying to explain this to his commanding office Bart Mancuso (Scott Glen) …

Jones: When I asked the computer to identify it, what I got was ‘magma displacement’. You see, sir, SAPS software was originally written to look for seismic events. And when it gets confused, it kind of ‘runs home to mama’.

Mancuso: I’m not following you, Jonesy.

Jones: Sorry, sir. Listen to it at times speed. [he plays a tape in which a rhythmic noise is heard] Now that’s gotta be man made, Captain.

Mancuso: Have I got this straight, Jonesy? A forty million dollar computer tells you you’re chasing an earthquake, but you don’t believe it? And you come up with this on your own?

Jones: Yes, sir.

Mancuso: Including all the navigational math?

Jones: Sir, I-I’ve got-

Mancuso: Relax, Jonesy, you sold me!

This is so true. That peak fitting software on your FTIR, WDS, EDS, EMP, XRF, whatever was probably not written with reference to archaeological materials. It was written to identify pure compounds used in pharmacy or precise mineral compositions.

So the lesson to learn here is if you get something unexpected, like a massive peak assigned to tellurium, you should probably question your results, not just accept it as something really unusual and therefore cool. Any scientists watching your presentation or reading your paper (or indeed reviewing it) will immediately see through this. Here is an example from the field of petrology. What you need to know before reading this is that basic mineral identification in rocks is carried out by (experienced) analysts using optical polarising light microscopy (PLM). So for example, a basalt may include a mineral from the pyroxene group where chemistry ranges between iron, magnesium and calcium and variable silica amounts. This group can be conveniently subdivided into two main groups orthopyroxenes and clinopyroxenes which have clear association with different geochemical environments. The ‘ortho’ and ‘clino’ bits refer to differences in crystal structure and these two groups are most easily distinguished using PLM. It would take about three seconds to distinguish the two pyroxenes and we generally just refer to them as orthopyroxene (opx) and clinopyroxene (cpx). If you really want to, you can further subdivide the pyroxenes into named types, say for cpx, the main ones are are augite, hedenbergite and diopside. However most of the time we geologists don’t use this classification unless there is a reason for doing so, i.e. we want to answer specific questions about zoning within a single crystal and how that relates to say, fluctuating melt chemistries (and we need a reason for deciding that fluctuating melt chemistries are important). We just call it clinopyroxene. Therefore when I see a presentation that shows someone has identified diopside in a pot sherd I know they haven’t looked properly at the material and a computer programme written for igneous petrologists has told them it is diopside and they didn’t question it. It is also important to note that the name ‘diopside’ does not denote a specific phase chemistry that will direct you to a particular source/provenance. It doesn’t and it won’t. And it isn’t ‘more scientific’.

peel-lake-sample-7-02

Clinopyroxene. I really DON’T CARE whether it is diopside, augite or hedenbergite.

On the subject of mineral names, the misuse that infuriates me most is the use of the mineral name cuprorivaite to describe the synthetic pigment Egyptian Blue. If you Google cuprorivaite, you will get far more references to Egyptian painting than to minerals and this demonstrates the scale of this problem. Cuprorivaite is a rare, naturally occurring mineral. When it occurs it is as a micromineral (very small crystals, < 1 mm) and often disseminated. It occurs nowhere in masses worth of economic extraction, even as a cottage industry. Therefore it has never been used as a pigment and no one has ever detected cuprorivaite on archaeological paintings. However, you would not believe this from the literature. What researchers have found is an analogous synthetic compound called Egyptian blue. It’s a calcium copper silicate if you want to sound scientific. Why is this terminology important? In the world of pigment analysis, there are many phases which can and have been used as pigments derived from natural minerals and analogous synthetic compounds and it is important in answering archaeological and art historical questions to be able to distinguish these forms. Examples are the red pigment mercury sulphide which when natural is cinnabar and when synthetic is vermillion, or (natural) azurite and (synthetic) verditer for the blue copper carbonate hydrates. It can be important to know the differences between these minerals to detect fakes and assess knowledge of technologies or mineral provenances. As a consequence it is essential to have a terminology that clearly differentiates between natural and synthetic pigments.

A photomicrograph of the mineral cuprorivaite.

I think a major problem here is that a lot of the time, scientists and cultural heritage people don’t really get each other. They don’t know enough about each others subjects. Sure, they watch the TV programmes and remembered being into the Ancient Egyptians at school, or they maybe had a toy chemistry set. But now we are all grown-up academics, we are all set in our ways and stuck in the silos that the university academic departments give us (and this structure is a problem). A chemist is probably not likely to meet an art historian at their institution unless they sit together on a college committee – and many academics will avoid these committees like the plague. So we don’t talk and learn about other disciplines, and don’t get me wrong, scientists are just as bad about this as anyone. Many can be arrogant, thinking that they can answer the ‘simple’ questions archaeologists ask and they think its cool to have something they have only seen before in a museum or on the TV in their lab. Something to tell their colleagues and hey, it might just tick some of those ‘impact’ boxes. So an art historian comes along with a Greek vase and the scientist thinks ‘OK I can analyse this for you using my synchrotron. This will cost hours of beam time and thousands of dollars, but we may get a paper from it that will enable me to patronise humanities people with my superior knowledge and show that I can also do novelty science. I have seen Greek vases in museums, I am surprised that no one has made these analyses before because it is so easy to do’. The art historian thinks ‘Brilliant, this will tell us something that all those other techniques won’t tell us. I don’t really understand those techniques, but surely this synchrotron thing is so big and so expensive that it must provide better, richer, more accurate and more precise results than anything else. This will make me look amazing when I present this work! So few people in my position have access to this sort of kit. Just using this technique makes this study novel and innovative. And the scientist guy says it will be easy. He seems to know what he’s doing.’. Maybe, just maybe, one of the parties will search the literature, but because scientific literature database on the whole doesn’t search books and memoirs and vice versa, they don’t find out that this is something that has already been done, using simple analytical techniques which have given better and more informative results.

And this happens within science disciplines too … this (modified) cartoon is on our geochron lab wall. ‘We’ are the geochronologists …

img_3669_2

Despite what you may interpret from the above, a geologist or geochemist would never use complex, expensive kit to do routine mineral analyses, so why would you do this to analyse the pigments on a wall painting or on a Greek vase? Many simple techniques such as microscopy and wet chemistry give reliable, accurate and precise results and can’t be improved on. The press releases surrounding the analysis of pigments on an Attic lekythos carried out on a synchrotron only told us that the white pigment present ‘contained calcium’. It stopped short of making the logical (i.e. totally f**cking obvious) observation that this was a form of calcite, a polymorph of calcium carbonate. If I could have taken a tiny sample from the vase (so small that it could not be noticed by the naked eye) I could have told the museum/art history guys that not only was it calcite, but would have been able to differentiate chalk, eggshell, seashell (OK the aragonite polymorph in this case), coral or any other form of calcite derived from a limestone. If sampling was not possible, I could have identified the presence of calcite/aragonite (very unlikely that it would be aragonite though) using a UV light torch and a pXRF in less than a minute. I would have probably charged £50 max for experimental costs. To present a paper on pigment analysis and conclude that you have a Ca-rich pigment or an Fe-rich pigment is simply not good enough and is not moving the subject on.

Mind you, on the subject of portable X-Ray fluorescence machines, these will only give semi-quantitative results and users should be aware of this, but they are good for identifying major elements when sampling is not possible. However pXRF analyses should not be used in isolation. These machines were designed for quick, rough, field checks to look at say, arsenic pollution in ground water. The manufacturers are horrified at how their data has been used to provide ‘confirmed scientific analyses’ in cultural heritage.

My examples here are mainly to do with geological and analogous materials but I am sure that many scientists can quote examples from metallurgy and biosciences where glaring misuse of equipment and terminology are used. How can this be improved? We all have to learn more from each other. Most importantly cultural heritage people need to know what questions they want to ask of their objects and materials. Is it as simple as ‘what is it?’ (a very valid question) or do they want to know more; where does it come from? How was it made? If there are more complex questions to be answered then find a scientist that can work with you to help find the best analytical method to get the answers you need.

My top tip is that scientists and cultural heritage partners work together in a truly collaborative manner, so that the scientists are not just used as technicians, they are an integral part of the research project. I have always worked in close partnership with colleagues and have made time to learn about the materials I am analysing and the contexts of the artefacts or building from which they are derived. I know about comperanda, I read the literature, I know what the expected range of materials are. I double check my results when I find something outside that. If its true I design further experiments or do field work to answer these new questions. I don’t just hand over the data and walk away. And I always, always use the most appropriate technique to perform scientific analysis. As a petrologist I am experienced with polarised light microscopy so that is my ‘go to’ technique. It is great for ceramics, mortars, pigments and of course, stone. I do accept that this takes learning and experience, but go on a course! Learn it! I do a lot of work on pigments so I use FTIR and Raman spectroscopy too. If I have questions about organic binders then I would turn to gas chromatography mass spectrometry, but as this is costly and wouldn’t go there unless it was really important. I use old-skool spot tests and wet chemistry for quick analyses of phases to detect things like lead or phosphate (there are loads of books on how to do this, and you can do it in you kitchen). I use pXRF and SEM/EDS, again for major elements only, but am cautious on how interpret results, knowing these are not quantitative techniques. I occasionally use X-Ray diffraction (XRD) to look at crystallinity in some minerals (mainly hematite and other iron oxides and iron oxide hydroxides in ochres). These techniques answer all the questions I can think of. I used a synchrotron once to try and design some experiments to further the understanding of the blackening of cinnabar/vermillion paints. It didn’t work.

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I used a synchrotron to analyse a pigment and all I got was this crappy picture.

Catch a falling star: the strange story of the Tocopilla Meteorite

Think ‘meteorites’ and think ‘art’ and the average human mind will probably conjure up the garish and probably fantastical cover of a sci-fi novel depicting colliding worlds in shades of pink and blue or alternatively an ‘artist’s impression’ of the late heavy bombardment or the precise moment of the Chicxulub impact. Think again.

Tate Britain is hosting a major retrospective of German artist Sigmar Polke’s (1941-2010) work, which is due to end this Sunday (8th February 2015; Herbreich et al., 2014). If you’re in London you should go and see it. Polke’s work spans decades and genres which at first present no surprises; 1960s Pop Art, for the 1970s, magic mushrooms and sex. And then you walk into rooms with a startling explosion of creativity influenced by Polke’s use of materials from the 1980s onwards. Here we find canvases glittering with mica, a large painting depicting a lump of gold ‘Goldklumpen’ (1982) painted with poisonous pigments orpiment and realgar (arsenic sulphides) and green copper arsenite pigments (Schweinfurt Green). Polke also experimented with even more dangerous substances such as uranium based pigments (i.e. Uranium [Pink], 1992); His intention here was to render his art more ‘harmful’.

Polke continuously experimented with materials. He became interested in ancient and traditional painting materials, even producing his own Tyrian purple extracted from seashells (with minor success) to create the painting Purpur (1986). He was also fascinated with modern pigments with interesting optical effects, predominantly composed of synthetic micas coated with thin layers of metal oxides. These included iridescent and metallic car paints and ‘Magic Purple’; the latter having the effect of appearing purple from one angle and then golden from another – this effect could be optimised by burnishing the painted surface, producing rather beautiful paintings including the triptych Negative Value I, II and III (1982).

Polke was fully cogniscent of the fact that many of his paintings would decay and would be beyond the help of the most skilled of painting conservators. He knew that his materials were difficult and would change over time. For example, the orange arsenic sulphide pigment realgar on Goldklumpen has altered to a yellow shade, indistinguishable from the orpiment. However that is not necessarily always going to be the case. One of his ‘pigments’ has remained unchanged for over 4 billion years, however this fact would not help any future forensic analyst of his paintings identify it.

In 1988 Polke produced asigpol006 series of five works entitled ‘The spirits that lend strength are invisible I-V’. Inspired by the ancient land and native peoples of America (Garrels, 2010), these works used a variety of materials and media, including powdered nickel, silver leaf and silver oxides, artificial resins and even Neolithic stone tools (in V, left). Perhaps most striking in terms of materiality is ‘The spirits that lend strength are invisible II’ which is a scatter of powdered meteorite dust dispersed onto artificial resin. This painting, currently on tour with the exhibition Alibis: Sigmar Polke 1963-2010 belongs to The Doris and Donald Fisher Collection.

polke meteorPolke used 1 kg of powdered Tocopilla meteorite to create this painting (right) which is somehow reminiscent of Palaeolithic cave art. Luckily Polke’s use of this material is well documented, otherwise for someone like me, who, perhaps a century from now, will be characterising the pigments used in late 20th Century art, this would come as an unexpected and unrecognisable surprise. It is an extra-terrestrial encounter with unfamiliar materials not found on Earth, except when delivered my meteorites.

The Tocopilla meteorite is one of many associated with the same bolide that was found in 1875 in the Antofagasta region of northern Chile. It exploded in the air before impact and 266 kg have subsequently been collected from a large area of which Tocopilla is just one locality. The find is now known collectively as North Chile (Grady, 2000) and the various and many fragments found have been shown to be chemically identical. Chemically, this is an iron meteorite, probably representing the core of a small planet that was smashed to bits during the early history of the Solar System. More specifically it belongs to a class of meteorites known as hexahedrites and predominantly composed of the iron-nickel alloy kamacite. These are iron-rich meteorites with only approximately 5-6% nickel present. It also contains 3400 parts per billion of the element iridium, which puts in in a class of hexahedrite meteorites known as IIAB (see Morgan et al., 1995).

Just what might a 22nd Century pigment analyst expect to find in this painting? Axon & Nasir (1982) analysed a sample of the 2.5 kg Tocopilla mass owned by the British Museum (now the Natural History Museum). They found that the main mineral kamacite formed large crystals composed of 94.51% iron, 5.05% nickel with a trace of cobalt (0.4%). Enclosed within these crystals were lath- or needle-shaped inclusions of another mineral called schreibersite, an iron-nickel phosphide, (Fe, Ni)3P. Schriebersite is brittle and the needle-shaped crystals form planar features known as ‘rhabdites’ along which the meteorite broke up into its many fragments. Nolze et al. (2006) found tiny inclusions of the chrome-nickel mineral carlsbergite in the schreibersite crystals. Also present are small, massive crystals composed of lamellae of extra-terrestrial sulphide minerals troilite (FeS) and daubréelite (FeCr2S4). None of these minerals would be encountered in terrestrial rocks. Iron does not occur native (even in alloys) naturally on our wet, oxygen-rich planet and although the iron sulphide pyrite (FeS2) is common on Earth, troilite is unknown from terrestrial sources.

SchreibersiteLeft, needles of schreibersite, included in kamacite, radiate out from an iron sulphide-rich grain. This image is adapted from Axon & Nasir (1982). The field of view is about 1 mm across.

Meteoritic material is in the majority ancient. With the exception of rare meteorites know to have been derived from either the Moon or Mars, the vast majority of meteorites solidified in the primordial Solar System. Morgan et al. (1995) used the rhenium-osmium geochronometer to calculate the age of the Tocopilla meteorite and others of similar composition. They found that it is the age of the earliest material known in our Solar System, over four and a half billion years old (4.5 Ga)., over a million years older than the oldest known minerals on Earth. Most meteorites sit in museums or museum archives or are bought and sold by collectors, but at least a part of the Tocopilla meteorite will endure in a most unexpected way on the walls of art galleries. Many casual observers would not know that ‘The spirits that lend strength are invisible II’ is constructed from a material older than our planet. Polke would have known this and therefore his painting’s resonance with deep time, almost beyond imagination. I think this would have satisfied him. Polke created an unique painting with a unique material legacy and one that takes materiality in art to new extremes.

Links & References

You can see some of Polke’s work on the MoMA website here http://www.contemporaryartdaily.com/2014/07/sigmar-polke-at-moma/

Tate http://www.tate.org.uk/whats-on/tate-modern/exhibition/alibis-sigmar-polke-1963-2010

Image: The spirits that lend strength are invisible II http://superficiecontextual.blogspot.co.uk/2009_07_01_archive.html

Image: The spirits that lend strength are invisible V http://shop.tate.org.uk/alibis-sigmar-polke-19632010/polke-the-spirits-that-lend-strength-are-invisible-v-custom-prints/invt/sigpol006

Axon, H. J. & Nasir, M. J., 1982, A microprobe study of Ni-Co distribution about a schreibersite body in the Tocopilla mass of the North Chile hexahedrite [BM 1931,13]., Mineralogical Magazine, 45, 283-284.

Garrells, G., 2010, http://www.sfmoma.org/explore/multimedia/audio/103

Grady, M. M., 2000, Catalogue of Meteorites: 5th Edition., Cambridge University Press. p. 371.

Halbreich, K., Godfrey, M., Tattersall, L. & Schaefer, M. (Eds.), 2014, Alibis: Sigmar Polke 1963-2010., Tate Publishing, London., 317 pp.

Morgan, J. W., Horan, M. F., Walker, R. J. & Grossman, J. N., 1995, Rhenium-osmium concentration and isotope systematics in group IIAB iron meteorites., Geochimica et Cosmochimica Acta., 59 (11), 2331-2344.

©Ruth Siddall 2015